AF tune technique without taking photos

Meh, there's too much rangefinder stuff in the Nikon description of the procedure so here's the description tailored for Canon bodies:

Enter Live View and establish critical focus on a high-contrast subject. Exit Live View.

Set the lens to MF (Manual Focusing). Be careful not to jostle the focus ring when you move the switch - you don't want to alter the focus you established in step #1.

Set AF tune to 0.

Look through the viewfinder to see if the camera thinks the subject is in focus by checking if the green focus confirmation dot is lit. You'll need to half-press the shutter (or AF-ON) while doing this to keep the metering/rangefinder from going to sleep. Assuming you get confirmation, increase the AF tune value and recheck the viewfinder, leaving the lens set to the original focus you established in step #1 - do not refocus the lens. You want to keep increasing the AF tune value until you no longer get a green confirmation dot. As you reach the edge of the range the green dot will either fail to light up or it may flicker...you may need to cycle the half-press of the shutter several times to confirm that the confirmation you get is consistent. You're done when you find the last positive AF tune value that gives you a consistent green-dot confirmation - that value represents the positive end of the AF tune range.

Return AF tune to 0 and repeat the procedure in step 4, but this time decreasing the value of AF tune (going negative). When done you've established the negative end of the AF tune range

Calculate your final AF tune value, which is the midpoint of the range you established in steps 5 and 6. For example if your range was -2 to +8 then the midpoint is +3. If your range was 0 to +12 then the midpoint is +6.

Set your AF tune value to your calculated midpoint. You're done.

Notes:

If you don't get a green confirmation dot at an AF tune of 0 then increase or decrease the AF tune value until you do. Your goal is to find the range of AF tune values that show focus confirmation - that range may be all positive or all negative, instead of the -/+ range described in the steps above.

If either end of the AF tune range (-20 or +20) still gives you focus confirmation, this means the range of AF tune values is wider than the camera's supported +/-20 AF tune range. Marianne Oelund came up with a clever workaround for this here.